1. Field of the Invention
This invention relates to an optical hydrogen gas detecting membrane which detects a hydrogen gas by measuring a change in the transmittance of light when exposed to an atmosphere containing hydrogen, the hydrogen gas detecting membrane using a thin film of platinum oxide.
2. Description of the Related Art
In recent years, global warming due to the emission of greenhouse gases (CO2, etc.) associated with mass consumption of fossil fuels has posed a problem. In response, the realization of an energy supply system relying minimally on fossil fuels has been necessitated. A system for electric power supply by a hydrogen fuel cell, in particular, is an electric power supply system which does not discharge CO2 as a greenhouse gas. Technologies for its construction are under study in many fields as an infrastructure for realizing hydrogen society aimed at sustained development.
However, hydrogen as a fuel is a combustible gas involving explosion, and its handling requires a careful safety measure. For this purpose, the development of a gas sensor or a gas detecting membrane, which safely detects a leaking trace hydrogen gas, becomes one of the most important challenges in realizing hydrogen society. Hydrogen sensors, which have been put to practical use, measure a change in electrical resistance on the surface of a semiconductor due to the adsorption of hydrogen to detect hydrogen. Since they embrace a power circuit which can become an ignition source for explosion, however, they have been problematic in terms of safety.
As a hydrogen detecting method which does not need a power circuit as a possible ignition source for explosion, there is a method using a hydrogen gas detecting adhesive tape comprising titanium oxide coated with a palladium oxide hydrate which is colored upon exposure to a hydrogen gas (see JP-A-8-253742). This tape has been proposed as a hydrogen detecting material capable of visual confirmation. However, it presents the problem that under ultraviolet radiation in an outdoor setting or the like, its sensitivity to detect a hydrogen gas declines because of the photocatalytic effect of titanium oxide.
Proposals have also been made for a hydrogen gas detecting tape using a hydrogen gas detecting coating pigment consisting essentially of fine tungsten oxide particles which are colored upon exposure to a hydrogen gas (see JP-A-2005-345338); and for optically detecting hydrogen sensors measuring the transmittance of light by a tungsten trioxide film, which is colored with a hydrogen gas, with the use of laser, a light emitting diode (LED) light source, and an optical detector such as a photodiode, in order to detect a hydrogen gas with high sensitivity (see JP-A-60-39536 and K. Ito and T. Ohgami, Appl. Phys. Lett. 60 (1992) 938).
A thin film of a metal oxide, such as tungsten trioxide, having palladium deposited thereon as a catalytic metal for dissociating a hydrogen molecule adsorbed to the surface into a hydrogen atom has the property of decreasing in the transmittance of light when touching an atmosphere containing hydrogen. Thus, it is the most promising candidate for a next-generation hydrogen detecting material.
To form a tungsten trioxide film which changes in optical characteristics in the presence of hydrogen, as described above, it is necessary to exercise subtle compositional control, such as the introduction of oxygen defects into tungsten trioxide, or crystal structure control such as amorphization. However, it has not been easy to form, with satisfactory reproducibility, tungsten oxide whose optical characteristics promptly change in response to a hydrogen gas. Thus, there has been a demand for a hydrogen detecting material which, when prepared, does not require control over the amount of oxygen defects.
The present invention has been accomplished in the light of the above-described circumstances. It is an object of the invention to provide a hydrogen gas detecting membrane whose safety is not problematic, whose manufacturing method is simple, and which can be prepared with satisfactory reproducibility.